This invention relates generally to an electrophotographic printing machine, and more particularly concerns an optical system having an arcuate screen for producing half-tone images of an original document.
A typical electrophotographic printing machine exposes a charged photoconductive member to a light image of an original document. The irradiated areas of the photoconductive member are discharged recording thereon an electrostatic latent image corresponding to the original document. A development system moves a developer mix of carrier granules and toner particles into contact with the latent image recorded on the photoconductive member. Toner particles are attracted electrostatically from the carrier granules to the latent image. In this manner, a powder image is formed on the photoconductive member. Thereafter, the powder image is transferred to a sheet of support material. After transfer, the sheet of support material passes through a fusing device which permanently affixes the toner powder image thereto.
A multi-color electrophotographic printing machine also employs this concept. However, in multi-color electrophotographic printing, each cycle is repeated a plurality of times, one for each discrete color being reproduced. In multi-color printing, the light image is filtered to record an electrostatic latent image on the photoconductive member corresponding to a single color. This single color electrostatic latent image is developed with toner particles of a color complimentary to the color of the filtered light image employed to form the latent image. Thereafter, the toner powder image is transferred to a sheet of support material. This multi-layered toner powder image is then permanently affixed to the sheet of support material forming a permanent color copy of the original document.
In most electrophotographic printing machines, tone gradations are difficult to form. To obviate this problem, screening methods are employed. Generally, a screening technique produces effect of tone gradation by variations in dot or line size. In the highlight zones, the dots or lines are small. These dots or lines increase in size through the intermediate shades until they merge together in the shadow region. At the highlight end of the tone scale, there will be complete whiteness while at the shadow end nearly solid black. The foregoing is described in U.S. Pat. No. 2,598.732, issued to Walkup in 1952. Other patents exemplifying various screening techniques are; U.S. Pat. No. 3,535,036, issued to Starkweather in 1970, U.S. Pat. No. 3,121,010, issued to Johnson et al. in 1964, U.S. Pat. No. 3,493,381, issued to Frosch in 1973, and U.S. Pat. No. 3,809.555, issued to Marley in 1974. Recently filed applications describing different screening techniques are copending application Ser. No. 511,976, filed in 1974, and copending application Ser. No. 507,169, filed in 1974.
It is well known that the illumination of an image point is in proportion to the COS.sup.4 of the solid angle between the illumination point and the image point. Thus, it can be seen that the illumination on a photoconductive surface will fall off quite rapidly as the solid angle increases. Various techniques have been devised to compensate for this effect. Typically, a sheet of opaque material having a butterfly slit formed therein is employed. The area of the slit is inversely proportional to the illumination profile. The butterfly slit is flat and the distance between the photoconductive surface and the slit varies. Thus, the light image projected through the slit, and, in turn the screen, travels differing distance to the photoconductive surface. This height variation results in copies having non-uniform density and color balance shifts.
It is the primary object of the present invention to improve the optical system of an electrophotographic printing machine by obtaining uniform copy density and color balance.